When a medium is illuminated by intense short pulses at some field strength, the medium is ionized and the electrons are accelerated in one half cycle of the field. Some of these electrons return back to the parent atom or molecule from which they originated and re-interfere with that part of the electron wavefunction that was not yet ionized, leading eventually to recombination of the accelerated electron under emission of an extreme ultraviolet (EUV or XUV) photon equivalent in energy to the sum of ionization energy and kinetic energy accrued during acceleration in the field. These energies typically cover the EUV to soft x-ray wavelength range from 10 eV to 1 keV and eventually even up to 10 keV. This process was named high-order harmonic generation (HHG).
Thus far, HHG has been thought to be a relatively inefficient generation process with efficiencies typically in the range of 10−6 to 10−8 or even less for energies above 45 eV. In the range below 40 eV, efficiencies of 10−5 were demonstrated in heavy gases such as Ar and Xe; but, in this range, the possibility of applications are limited.
Previous HHG studies were pursued with titanium (Ti):sapphire lasers at 800 nm because those lasers enable the generation of the shortest pulses directly from laser-based oscillators and amplifiers. For fundamental studies, sometimes the second harmonic of Ti:sapphire, 400 nm light, or a combination of 400 and 800 nm light was used. More recently, to demonstrate very-short wavelength EUV generation, longer wavelength driver pulses with wavelengths of 1.6 micron and even 2 micron have been used or proposed.
Current technology for EUV sources for EUV lithography is based either on discharge-produced plasma (DPP) sources or laser-produced plasma (LPP) sources. It is expected that the first generation of EUV-lithography production tools will likely use LPP sources. For future technologies, it is thought likely that the source power requirements will even increase to more than 500 W. Current source demonstrations for both DPP and LPP sources are below 50 W.
EUV source readiness is still seen as the major risk to introducing EUV lithography. In addition, LPP sources have the undesired effect of debris production damaging the source optics. Also, both DPP and LPP sources are not spatially coherent; therefore, EUV light collection is further problematic and a source for low efficiency. In contrast, HHG sources are spatially fully coherent sources producing a well-collimated beam.
The use of passive enhancement cavities for coherent pulse addition and EUV generation was proposed in US Patent Application Publication No. 2006/0268949 A1 and demonstrations have been reported in C. Gohle, et al., “A Frequency Comb in the Extreme Ultraviolet,” 436 Nature 234-37 (2005) and in R. Jones, et al., “Phase-Coherent Frequency Combs in Vacuum Ultraviolet via High-Harmonic Generation Inside a Femtosecond Enhancement Cavity,” 94 Phys. Rev. Lett. 193201 (2005). However, these early studies were not for the purpose of efficiency enhancement but rather for generation of frequency combs for high-resolution laser spectroscopy at an EUV wavelength less than 50 eV. In this early work, output coupling of the EUV light from the cavity was problematic. The use of cavity mirrors with holes and the use of LP01 Laguerre Gaussian modes were tried with little apparent success.
Output coupling with a sapphire Brewster plate, which reflects the EUV but transmits the laser radiation, is currently used with the disadvantages that the plate is relatively easily damaged and the plate nonlinear index leads to undesired cavity resonance shifts. More recently, a plate with an EUV micrograting was used for output coupling, which, however, shows only a limited output coupling efficiency of 10%, D. Yost, et al., “Efficient Output Coupling of Intracavity High-Harmonic Generation,” 33 Opt. Lett. 1099-1101 (2008). Another problem is damage to the cavity mirrors due to the high intensities in the HHG process.